Conventionally, in a sound-intensifying system for intensifying a sound signal collected by a microphone, a howling suppression device, for detecting an occurrence of howling and suppressing the howling, has been developed. As a conventional howling suppression device, a howling suppression device using an application filter or a notch filter is well-known (see patent document 1 and patent document 2, for example).
Hereinafter, with reference to FIG. 10, a sound-intensifying system, for receiving a plurality of sound signals, and mixing the plurality of sound signals to be intensified, in which the conventional howling suppression device is adopted, will be described. FIG. 10 is a view illustrating an exemplary configuration of a sound-intensifying system 9, for mixing and intensifying the plurality of sound signals, in which the howling suppression devices disclosed in patent document 1 and patent document 2 are adapted. Note that FIG. 10 shows the exemplary configuration of the sound-intensifying system 9 for suppressing howling to be occurred when a speaker and a plurality of microphone are in the same sound field. Here, as the plurality of sound signals, it is assumed that two sound signals are inputted from two microphones.
In FIG. 10, the sound-intensifying system 9 includes a first microphone 91a, a second microphone 91b, a sound characteristic adjusting section 92, a sound mixing section 93, a howling suppressing section 94, and a speaker 95. The sound characteristic adjusting section 92, to which a sound signal collected and generated by the first microphone 91a is inputted, adjusts a frequency and gain characteristic of the sound signal. Similarly, the sound characteristic adjusting section 92 adjusts a frequency and gain characteristic of a sound signal collected and generated by the second microphone 91b. Thereafter, each of the adjusted sound signals are mixed by the sound mixing section 93. Note that the sound characteristic adjusting section 92 and the sound mixing section 93 correspond to a commercially available mixer shown in FIG. 11, for example. FIG. 11 is a block diagram illustrating an exemplary configuration of the sound characteristic adjusting section 92 and the sound mixing section 93. In FIG. 11, the sound characteristic adjusting section 92 includes an equalizer 921a, an equalizer 921b, an amplification section 922a, and an amplification section 922b, for example. The equalizer 921a adjusts the frequency characteristic of the sound signal collected and generated by the first microphone 91a. The amplification section 922a adjusts the gain characteristic of the sound signal adjusted by the equalizer 921a. Similarly, the equalizer 921b and the amplification section 922b adjust the frequency characteristic and gain characteristic of the sound signal collected and generated by the second microphone 91b. As described above, similarly to the commercially available mixer, in the sound characteristic adjusting section 92, the frequency characteristic and gain characteristic of the sound signal collected by the first microphone 91a and the frequency characteristic and gain characteristic of the sound signal collected by the second microphone 91b are adjusted in an independent manner. The sound signal mixed by the sound mixing section 93 is inputted to the howling suppressing section 94.
The howling suppressing section 94 performs a signal processing on the sound signal mixed by the sound mixing section 93 so as to suppress howling. Thereafter, the sound signal on which the signal processing has been performed is amplified as necessary so as to be outputted by the speaker 95. Note that the howling suppressing section 94 corresponds to a howling suppression device for suppressing the howling. As described above, in this example, the sound-intensifying system adopts howling suppression methods disclosed in patent document 1 and patent document 2. Thus, an application filter or a notch filter is used as the howling suppressing section 94.
FIG. 12 is a block diagram illustrating an exemplary configuration of the howling suppressing section 94 in which an application filter 941 is used. In this case, based on the sound signal (the sound signal to be intensified) outputted from the howling suppressing section 94, the howling suppressing section 94 estimates, only when the sound signal is outputted therefrom, a transfer characteristic such as a spatial transfer characteristic. Thereafter, the application filter 941 multiplies the estimated transfer characteristic by the sound signal to be intensified, and subtracts the multiplied transfer characteristic from the sound signal outputted from the sound mixing section 93, thereby making it possible to suppress a howling occurrence.
Alternately, the notch filter may be used as the howling suppressing section 94. FIG. 13 is a view illustrating a change in a power spectrum X(ω) of the sound signal outputted from the sound mixing section 93 at a time of the howling occurrence. It is assumed that howling occurs, for example, at a specific frequency f. In this case, the power spectrum X(ω) shown in FIG. 13 changes such that power of the power spectrum rapidly increases at the specific frequency f. Therefore, a power difference between a frequency band and its adjacent frequency band is always monitored, thereby detecting that power in a frequency band including the specific frequency f is rapidly increased. That is, a frequency at which the howling occurs can be detected. In this case, a frequency to be attenuated by the notch filter is set at the specific frequency f. Then, the sound signal outputted from the sound mixing section 93 is passed through the notch filter which attenuates the sound signal at the specific frequency f, whereby the power at the specific frequency f is to be attenuated. As a result, a howling occurrence is to be suppressed.    [Patent document 1] Patent publication No. 2039846    [Patent document 2] Patent publication No. 2560923